Prior systems that measure the amount of fuel in a fuel tank typically use a mechanical system comprising a variable resistor associated with a float. The resistance of the variable resistor is modified depending upon the upward and downward movement of the float. However, such mechanical systems tend to be unreliable and are prone to breakage over time.
One approach to solve this problem is disclosed in U.S. Pat. No. 3,693,445 by Sven Johnson. This patent teaches a "solid state" measurement system that employs an ultrasonic transducer. The system is analogous to sonar, in that, the round trip time for a signal pulse is used to deduce distance. The system includes an ultrasonic transducer, a tube, a float and associated electronic circuitry.
The float is disposed inside the tube and floats on the surface of the liquid. The ultrasonic transducer is mounted on the bottom of the liquid tank and directs ultrasonic pulses toward the float. For example, a pulse is generated by the electronic circuitry is transmitted acoustically through the liquid by the transducer. A portion of the ultrasonic pulse is reflected back to the transducer by the float and is detected by the electronics. However the system disclosed by Johnson has several problems; a few of such problems will be discussed here.
One problem pertains to the float, which is disclosed as a hollow steel ball. As shown by FIG. 1 of the Johnson patent, the ball has a relatively small radius as compared to the diameter of the tube. Thus, the resulting spherical surface of the ball is not at a uniform distance to the ultrasonic transducer. Therefore, parts of the transmitted ultrasonic pulse will contact the ball surface at different instances in time. This causes the intensity of the reflected pulse to reduce, as well as, the reflected pulse to distort; thereby, resulting in inaccurate measurements.
Another problem pertains to the ultrasonic transducer itself. Typically, ultrasonic transducers comprise a single piezoelectric disc. As well known in the art, the disc expands in response to receiving electrical energy (transmission), and conversely the disc produces electrical energy in response to receiving mechanical energy (reception). However in order to produce accurate measurements, complex damping schemes are needed to dampen the sensor in between the transmission and reception of an ultrasonic pulse to prevent false measurements. This is especially needed when the liquid level is very low, which results in a quick round trip time. Unfortunately such damping schemes include complex electronic damping circuitry or mechanical damping configurations that add excessive cost to the system.
Finally, the system of Johnson does not teach compensating for variations in the speed of the ultrasonic pulse as the temperature of the liquid changes. It is well known that changing temperatures cause the material properties of a liquid to change; thereby, altering the speed of the ultrasonic pulse. Since vehicles are exposed to harsh environmental conditions such as extreme heat and cold, a fuel level measurement system for a vehicle must account for changes in temperature to provide accurate measurements.
The present invention is directed to overcoming one or more of the problems as set forth above.